The present invention concerns a device for measuring a bending load in constructions, according to the introductory part of claim 1.
The present invention is based upon the principle of using a fibre optic Bragg grating. A Bragg grating is a single mode fibre having permanent periodical variation in the refraction index over a fibre length of for example 0,1 to 10 cm. Variation in the refraction index is established by illuminating the fibre with an UV-laser. A bragg grating reflects light at a wave length given by the refraction index and the period related to space for the variation in refraction index (grating period), while light outside this wave period will pass the grating more or less unhindered. The light reflected by the Bragg grating will give a wave length which varies as a function of a measuring dimension changing the refraction index of the fibre material in the grating and/or by the fibre length in the grating zone (the grating period). Tension in the fibre or temperature will thus give a change in wavelength for the light reflected in the Bragg grating.
In practical use, temperature can be measured in the range xe2x88x92100xc2x0 C. to +250xc2x0 C. at approximately 20 points along a fibre having a length of 50-100 km. Using various multiplexing techniques, the number of measurement points can be increased. Examples of areas of application are temperature surveillance of power cables, pipelines, electrical transformers, engines and temperature monitoring of industrial processes.
A number of devices for measurement of bending in mechanical constructions exist. For special purposes where there is little space available or there is high temperature, high tension and so forth, all known devices for measurement of bending load have functional disadvantages. For example, measuring bending under water is made with bending sensitive sensors based on electrical elements, which in such environments exhibit low reliability. For other areas of application there may be little space available for installing extra components, such as bend sensors based on electrical induction or capacity (typical diameter 10-20 mm). Another problem with sensors based upon electrical effects is electrically induced noise. For example lightning strikes have sometimes rendered sensor elements or electronic circuits passive, and thus disabled the bend monitoring.
Accordingly, there is a need for a bend sensor with mainly passive components, that can be utilized in difficult environments and restrictive spaces.
The objective of the present invention is to provide a device for measuring bending in and on mechanical constructions.
The objective is achieved with a device according to the characterizing part of Claim 1. Further features are disclosed in the dependent claims.
The invention relates to a device for measuring bending in mechanical constructions, the device comprising:
a first sensor for measuring a bending load, said sensor including a housing connected to an optical fibre mounted/prestressed in a first anchoring point and a second anchoring point by the housing, wherein the fibre is provided with a first Bragg grating, located in the prestressed fibre, the housing being arranged so that the fibre, when exposed to a strong bending force on the housing, is not brought in contact therewith and is exposed to tension load,
a second sensor for measuring tension load, said sensor including a housing connected to an optical fibre prestressed in a first anchoring point and a second anchoring point by the housing, wherein the fibre is provided with a second Bragg grating, located in the mounted/prestressed fibre, the housing being arranged with an element forcing the fibre to bent along with the housing,
a third sensor for measuring temperature, said sensor including a housing attached to an optical fibre suspended in a first anchoring point and a second anchoring point by the housing without prestressing the fibre, wherein the fibre includes a third Bragg-grating located in the suspended fibre, the housing being arranged so that the fibre, by a strong bending or tension of the housing, is not brought into contact therewith and is exposed to tension load,
wherein one or more of the respective sensors are arranged sequenteally in the housing along the common optical fibre.
The Bragg-grating in the first sensor will be influenced by bending, tension and thermal effects. The Bragg-grating in the second sensor will mainly is only influenced by tension and thermal effects (for example expansion in the second sensor), while the Bragg-grating in the third sensor only is influenced by thermal effects. By combining two or three of the sensors described above, effects from tension and temperature are compensated so that the device measures correct bending forces. All sensors can be integrated into the same housing.
The principal design of a bend measuring sensor according to the present invention renders it possible to produce bend measuring sensors with very small dimensions capable of measuring both small and large bending radii from distant positions. The device also has the capability of measuring bending in different positions along the same optical fibre.
Examples of mechanical constructions are constructions where the present invention is applicable include bridges, flexible pipes and the like.